Potential Effects of Operating a Terrestrial Radio Network Near GPS Frequency Bands Assessed by New Report
A new congressionally mandated report from the National Academies of Sciences, Engineering, and Medicine assesses the likelihood of harmful interference from Ligado Networks’ authorized use of the radio frequency spectrum near bands used by GPS and other satellite services.
The report says use of radio frequency bands — as authorized by the Federal Communications Commission in its 2020 order — by Ligado Networks, a private satellite communications company, will not cause most commercially produced general navigation, timing, cellular, or certified aviation GPS receivers to experience harmful interference. However, some high-precision receivers, used for applications such as farming, geodesy, and surveying and sold before about 2012, can be vulnerable to significant harmful interference. In addition, certain mobile satellite services provided by Iridium Communications Inc. and used by the U.S. Department of Defense and others will experience harmful interference under certain conditions.
The committee that wrote the report reached these conclusions from its review of signal transmission and reception physics and engineering pertaining to devices working in the spectrum region in question, as well as from publicly available interference tests evaluating various operating scenarios and representative devices.
The radio frequency spectrum is a natural resource that underpins all wireless activity, from mobile phones to GPS and satellite communications. The spectrum is divided into frequency bands, each allocated for specific services and managed in the United States by two agencies — the Federal Communications Commission and the National Telecommunications and Information Administration. While strict adherence to transmission and reception only within authorized frequency bands would provide protection against harmful interference, in practice transmitters do not have sharp cutoffs and may emit signals beyond their authorized frequencies, and receivers may pick up frequencies beyond their authorized range. Both of these factors can potentially contribute to interference that degrades performance or causes loss of operation.
Due to the commercial, economic, and national security interests involved, and questions over whether signals from Ligado transmitters could create adverse effects including the disruption of critical services, Congress, through the DOD, requested a study from the National Academies. The committee’s statement of task did not include consideration of whether the FCC reached a correct outcome in authorizing the Ligado system.
Manufacturers are technologically capable of building new GPS receivers that are robust enough to withstand interference from Ligado Networks’ signals, the report says. The installation of new antennas and electronics, as well as the extension of exclusion zones in which Ligado would not be permitted to operate, offer other potential avenues for mitigating harmful interference. In some cases, though, these mitigation procedures may not be practicable at relevant time scales or reasonable costs due to test certifications and other efforts required.
Additionally, the report finds that neither of the two prevailing approaches for evaluating harmful interference concerns in the radio spectrum effectively mitigates the risk of harmful interference. While assessments of positional accuracy degradation and signal-to-noise ratio reduction both have a role to play, there are multiple ways in which interference can degrade receiver operations that are not considered by these two approaches. These approaches also require cumbersome device-by-device testing, and lack analytical, repeatable, and straightforward criteria, thus impeding progress in making more efficient and effective use of the spectrum.
The report says that a more collaborative approach to resolving spectrum issues, joint study and testing by the FCC and NTIA of proposed changes to spectrum use, more definitive receiver performance standards, and the establishment of specific timespans where adherence to those standards will ensure successful operation are all important tools for the future of the sector.
“Our technological capabilities in this space are constantly evolving, so spectrum real estate should be seen as a living asset that evolves alongside new technologies,” said J. Michael McQuade, strategic adviser to the president at Carnegie Mellon University and chair of the committee. “But to ensure stability, approaches must also allow for a degree of confidence that a deployed system will not be compromised, for a time, by harmful interference from new entrants.”
The committee evaluated “harmful interference” as related to relevant physics and engineering questions, using the term in a general sense to imply degraded receiver operations. It did not assess whether “Harmful Interference” as defined by U.S. or international regulations, which include language pertaining to assessments of the consequences of the interference, might occur.
A classified annex to the report provides further information on the test results and analysis as they relate to DOD systems and missions.
The study — undertaken by the Committee to Review FCC Order 20-48 Authorizing Operation of a Terrestrial Radio Network Near the GPS Frequency Bands — was sponsored by the U.S. Department of Defense. The National Academies of Sciences, Engineering, and Medicine are private, nonprofit institutions that provide independent, objective analysis and advice to the nation to solve complex problems and inform public policy decisions related to science, technology, and medicine. They operate under an 1863 congressional charter to the National Academy of Sciences, signed by President Lincoln.
Contact:
Josh Blatt, Media Associate
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202-334-2138; e-mail jblatt@nas.edu
